Show HN: Fursoy – Out-of-order RV64IMAFDV core in SystemVerilog (Linux 6.6)

Fursoy Slice-1

A 64-bit Out-of-Order Superscalar RISC-V Processor (RV64IMAFDV + Zba + Zbb + Zicsr + Zifencei) booting mainline Linux 6.6.50 and BusyBox userspace under Sv39 virtual memory.

Fursoy Slice-1 (fursoy_s1_core / fursoy_s1_soc) is a clean-slate 64-bit out-of-order superscalar RISC-V processor implemented in SystemVerilog, featuring Sv39 demand paging, machine/supervisor/user privilege separation, cycle-exact hardware performance telemetry, and 500 MHz Nangate 45nm standard-cell synthesis closure across 8 core functional blocks.

Microarchitecture Highlights

• 2-Wide Superscalar OoO Core (fursoy_s1_core): 2-wide fetch, decode, register rename, and in-order retirement (M/S/U privilege modes, medeleg/mideleg, WFI, MRET/SRET). 128-entry Reorder Buffer (ROB_DEPTH = 128) with 4-bit wrap-safe epoch tags and precise exception/interrupt flush recovery. 160-entry Physical Integer Register File (PRF_SIZE = 160, 128 speculative free-list entries) plus 8 branch checkpoints (NUM_CHECKPOINTS = 8) for single-cycle speculative RAT rollback. Dedicated issue queues: 32-entry Integer IQ (INT_IQ_DEPTH = 32), 32-entry Memory IQ (MEM_IQ_DEPTH = 32), FP IQ (FPRF_SIZE = 64), and decoupled 8-entry Vector IQ (VLEN = 256, ELEN = 64, 48 vector physical registers). Parallel-prefix 64-bit Kogge-Stone fursoy_s1_alu and fursoy_s1_branch_unit (single-cycle execution + carry-free target equality), 3-stage pipelined multiplier, and hardware divider.

• 2-wide fetch, decode, register rename, and in-order retirement (M/S/U privilege modes, medeleg/mideleg, WFI, MRET/SRET).

• 128-entry Reorder Buffer (ROB_DEPTH = 128) with 4-bit wrap-safe epoch tags and precise exception/interrupt flush recovery.

• 160-entry Physical Integer Register File (PRF_SIZE = 160, 128 speculative free-list entries) plus 8 branch checkpoints (NUM_CHECKPOINTS = 8) for single-cycle speculative RAT rollback.

• Dedicated issue queues: 32-entry Integer IQ (INT_IQ_DEPTH = 32), 32-entry Memory IQ (MEM_IQ_DEPTH = 32), FP IQ (FPRF_SIZE = 64), and decoupled 8-entry Vector IQ (VLEN = 256, ELEN = 64, 48 vector physical registers).

• Parallel-prefix 64-bit Kogge-Stone fursoy_s1_alu and fursoy_s1_branch_unit (single-cycle execution + carry-free target equality), 3-stage pipelined multiplier, and hardware divider.

• Sv39 Virtual Memory & Non-Blocking Caches: Dual-port Sv39 DTLB: 64-entry 4-way set-associative base TLB (16 sets, Tree-PLRU) + 8-entry 2 MiB / 1 GiB superpage CAM with 32-entry Translation Request Table (TRT_DEPTH = 32). Sv39 ITLB: 64-entry 4-way set-associative base TLB + 8-entry superpage CAM. Shared Hardware PTW (fursoy_s1_ptw): Walks Sv39 page tables with non-destructive L1D dirty snoop coherency so page-table writes sitting dirty in L1D are observed directly without eviction, paired with SFENCE.VMA store-queue drain and D-cache clean synchronization. L1I Cache (fursoy_s1_l1i): 16 KiB, 4-way set-associative (64 sets × 64 B), VIPT-safe with conservative next-line prefetcher and 2 I-MSHRs. L1D Cache (fursoy_s1_l1d): 16 KiB, 4-way set-associative (64 sets × 64 B), 8-bank (PA[5:3]) non-blocking data cache with 16-entry Load Queue, 16-entry Store Queue, 4 MSHRs, and 2-entry Writeback Buffer.

• Dual-port Sv39 DTLB: 64-entry 4-way set-associative base TLB (16 sets, Tree-PLRU) + 8-entry 2 MiB / 1 GiB superpage CAM with 32-entry Translation Request Table (TRT_DEPTH = 32).

• Sv39 ITLB: 64-entry 4-way set-associative base TLB + 8-entry superpage CAM.

• Shared Hardware PTW (fursoy_s1_ptw): Walks Sv39 page tables with non-destructive L1D dirty snoop coherency so page-table writes sitting dirty in L1D are observed directly without eviction, paired with SFENCE.VMA store-queue drain and D-cache clean synchronization.

• L1I Cache (fursoy_s1_l1i): 16 KiB, 4-way set-associative (64 sets × 64 B), VIPT-safe with conservative next-line prefetcher and 2 I-MSHRs.

• L1D Cache (fursoy_s1_l1d): 16 KiB, 4-way set-associative (64 sets × 64 B), 8-bank (PA[5:3]) non-blocking data cache with 16-entry Load Queue, 16-entry Store Queue, 4 MSHRs, and 2-entry Writeback Buffer.

• SoC & Peripherals (fursoy_s1_soc): 5-to-1 Priority Crossbar: Core MMIO > PTW walks > L1I refills > L1D refills > L1D dirty writebacks, with anti-starvation fairness and unmapped-aperture bus error trapping. Platform Devices: 256 MiB DRAM interface (0x8000_0000), CLINT (0x0200_0000), PLIC (0x0C00_0000, 31 external sources), 16550 UART (0x1000_0000), and custom minimal SBI v2.0 M-mode firmware (Base, TIME, SRST, DBCN).

• 5-to-1 Priority Crossbar: Core MMIO > PTW walks > L1I refills > L1D refills > L1D dirty writebacks, with anti-starvation fairness and unmapped-aperture bus error trapping.

• Platform Devices: 256 MiB DRAM interface (0x8000_0000), CLINT (0x0200_0000), PLIC (0x0C00_0000, 31 external sources), 16550 UART (0x1000_0000), and custom minimal SBI v2.0 M-mode firmware (Base, TIME, SRST, DBCN).

Physical Synthesis & STA (Nangate 45nm @ 500 MHz)

Evaluated standalone across 8 core functional blocks using Yosys 0.33 + sv2v v0.0.13 + OpenSTA 3.1.0 on NangateOpenCellLibrary (45nm Typical, 1.10V, 25C) targeting 500 MHz (2.000 ns clock period) (tools/run_s1_synth.py --run-synth, full report in build/synth/s1_synth_report.json):

Note: Total standard-cell area across these 8 standalone blocks is 175,106.74 um^2 (NAND2_X1 = 0.798 um^2). Full-SoC place-and-route and SRAM macro compilation are not yet executed.

Linux 6.6.50 Boot & Single-Owner CPI Telemetry

The Milestone 9 closeout (tools/run_s1_m9_closeout.py, saved in results/s1-m9-closeout.json) boots mainline Linux 6.6.50 in Verilator with zero kernel panics, zero oopses, zero access faults, and 0 CPI conservation errors across 89,861,075 simulated cycles (76,236,390 retired instructions, 2,227,761 U-mode commits, 2,255 demand page faults / 1,027 U-mode, IPC = 0.84838):

        [    0.000000] Linux version 6.6.50 ... (riscv64-linux-gnu-gcc 13.3.0)
[    0.000000] SBI specification v2.0 detected
[    0.000000] SBI implementation ID=0xf00501 Version=0x1
[    0.000000] earlycon: uart8250 at MMIO32 0x0000000010000000 (options '')
...
[    6.809584] Run /init as init process
[INIT] Fursoy Slice-0 Linux BusyBox Userspace active!
FURSOY_USERSPACE_BEGIN
[TEST 01] uname ... Linux (none) 6.6.50 riscv64 GNU/Linux
...
[TEST 20] repeated process creation/exit
[TEST 21] userspace memory stress
[MEM_STRESS_START] Running stack and heap multi-width memory validation...
[MEM_STRESS_PASS] All stack, heap, multi-width, and strided tests passed.
FURSOY_USERSPACE_PASS
      

Cycle-Exact 8-Category CPI Conservation Breakdown

Every cycle is mutually exclusively owned by one architectural bucket asserted continuously in hardware (sum(C_k) == mcycle = 89,861,075, cpi_conservation_errors = 0):

Quickstart (Prerequisites & Verification)

Prerequisites (Ubuntu 24.04 / WSL2)

        sudo apt-get update
sudo apt-get install -y \
  verilator g++ python3 device-tree-compiler \
  gcc-riscv64-unknown-elf gcc-riscv64-linux-gnu
      

(Optional, for physical synthesis & STA: yosys, sv2v, OpenSTA sta, and Nangate45 .lib via FURSOY_LIBERTY or tools/synth/lib/NangateOpenCellLibrary_typical.lib.)

Running Verification

From the repository root:

        # 1. Production RTL lint (Verilator --lint-only, expects 0 warnings/errors)
python3 tools/run_s1_lint.py

# 2. Fast Slice-1 verification regressions (runtime ~3-6 min total)
python3 tools/run_s1_m9_phase_a.py   # Interrupts, mideleg, WFI, precise trap boundaries
python3 tools/run_s1_m9_phase_b.py   # CLINT, PLIC, UART 16550 platform peripherals
python3 tools/run_s1_m9_phase_c.py   # 5-master crossbar, DTB, unmapped faults
python3 tools/run_s1_m9_phase_d.py   # SBI v2.0 firmware + mixed-privilege stress
python3 tools/run_s1_m8_closeout.py  # RV64FD + RVV 1.0 FP/Vector backend
python3 tools/run_s1_m7_closeout.py  # Multi-bank non-blocking L1D + coherency
python3 tools/run_s1_m6_closeout.py  # Frontend BPU / ITLB / L1I NLP

# 3. Nangate 45nm @ 500 MHz synthesis & OpenSTA check
python3 tools/run_s1_synth.py --run-synth

# 4. Full Linux 6.6.50 + BusyBox boot closeout (~1.5-2 hours wall time)
# Requires a built RISC-V Image at build/linux-6.6.50/arch/riscv/boot/Image
# (kernel config/patches in software/linux/, initramfs helper in tools/build_initramfs.py).
python3 tools/run_s1_m9_closeout.py
      

Repository Layout

License

Licensed under the Apache License, Version 2.0.